Instruments that utilize focused electron beam include imaging instruments, such as scanning electron microscope (SEM) and transmission electron microscope (TEM); manufacturing instruments such as electron beam lithography machine (EBL) and chemical analysis instruments such as energy dispersive x-ray spectrometer, electron energy loss spectrometer (EELS), and auger electron spectrometer. The expectation for these instruments to achieve a higher performance requires an electron source that is capable of providing brighter electron beam with narrower energy spread. The brightness expectation is for achieving a higher signal-noise ratio for analytical instruments and high throughput for lithography machines; The narrower energy spread expectation is for better focusing power with the electromagnetic lens, since a finite chromatic aberration is inevitable in the current level of lens manufacturing. The brightness and energy spread of the electron source depend on both the electron source material and how the electron beam is generated.
In order to have electrons liberated from the electron source surface, which is termed as an emitter, one needs to provide the bound electrons with enough energy to overcome the energy barrier between the emitter surface and vacuum. The height of this energy barrier is defined as the emitter material work function. If thermal heating is used to excite the bound electrons to overcome this barrier, the emission mode is termed as thermionic emission and the electron source is termed as thermionic emitter. When the work function of the emitter material is lower, it requires lower temperature for the same degree of thermal excitation, which results in both an increase in emission brightness and a reduction in energy spread. Typical low work function thermionic emission materials include borides, carbides and oxides of elements belonging to the group 2, 3 and 4 in the periodic table. When heated to the same temperature, these low work function materials emit higher current density of electrons compared to the conventional thermionic emitter material W, which has a high work function. When one applies a negative electric potential on the emitter with respect to a neighboring electrode, the energy barrier height is reduced with increasing electric field strength, due to the schottky effect. This lowering of the barrier height helps electrons to escape the emitter surface with greater ease when thermal heating is still present for electron excitation. This emission mode is termed as schottky emission or field-assisted thermionic emission and the emitter is termed as schottky emitter. With an emitter material of ZrO/W (with work function of ˜2.6 eV), schottky emitter produces higher brightness and lower energy spread than thermionic emitters. Yet when one keeps increasing the applied field strength, the energy barrier becomes so thin that the bound electrons can directly tunnel through the barrier into vacuum even without any thermal excitation. This emission mode is termed as field emission and the emitter is termed as field emitter. The relaxation from the requirement for high temperature (>1800K) thermal heating enables the field emission mode with the highest brightness and lowest energy spread among the three modes of electron emission. With the same electric field strength, it follows that the lower work function of the emitter material, the thinner the energy barrier becomes, therefore greater ease of electron tunneling. This results in a higher brightness and lower energy spread for field emitter with a lower work function than that with a higher work function.
Another important aspect for a practical electron emitter is that it should be able to emit electrons with current density unchanged over long period of time. A fluctuating or decaying/growing current adds complexity to the application instrument design and use. One key factor that causes emission current change is the adsorption of residue gas molecules left in the imperfect vacuum by the field emitter. The adsorbed molecules change surface work function and therefore change emission current density. For a thermionic emitter, the adsorption effect is less of an issue as compared to the low temperature field emitter. This is because the high temperature adopted in the thermionic emission process thermally desorbs any adsorbates from the surface and therefore maintains the same surface work function. For a low temperature field emitter, since there is no mechanism to drive off these adsorbates, an emission current fluctuation and decay over time is the consequent outcome. To lessen the influence from adsorbates, it is intuitively to create better vacuum where the residue gas is less. However, the requirement for higher vacuum adds up instrument cost and is also in sacrifice of operation convenience.
The currently commercialized field emitter is W with a high work function (4.5 eV). The high work function limits the highest achievable brightness with a tolerable energy spread for use in a focused electron beam instrument. On top of that, the W emitter is also known to be reactive to residue gas (presumably hydrogen) in vacuum. Its pre-decay plateau period is usually below 30 minutes in a vacuum not better than 1E-10 torr and below 5 hours in a vacuum not better than 1E-12 torr. In comparison, a schottky emitter usually emits electrons without decay in a vacuum not better than 1E-9 torr. The shorter stable period and higher vacuum requirement have made low temperature field emitter unpopular though it provides a higher brightness and narrower energy spread as compared to the schottky emitter. It is therefore highly desirable to engineer a low temperature field emitter material with low work function and high surface inertness to work stably for longer period of time under poorer vacuum condition.
It is generally believed that oxides are more chemically inert than metals of its composing element. However, oxides alone cannot be used as low temperature field electron emitters, because their conductivity is too low for electric current transportation. The structure of a thin film of low work function oxide over a conducting substrate can transport electrons sufficiently well to support low temperature field emission from the oxide surface. The requirement to make a stable structure is that the oxide layer has to be strongly bonded to the conductive substrate to endure the high electrostatic force generated by the extraction voltage during field emission process. The vibration or breaking off of the oxide layer will introduce field emission current fluctuation and decay. Usually, for a specific substrate material, there is only one specific crystal plane (or several planes) that it forms strong bonds with a certain oxide material. Therefore, the substrate needle has to be made in the form of a single crystal oriented along that specific crystal direction so that it has a tip top surface strongly bound with a layer of oxide film. Structure like this also benefits from a collimated beam shape, because the emission site is localized to that specific crystal plane, due to the fact that only that plane has a lowered work function. In the prior art, it has been found that the oxide of Zr preferably bound to the (100) surface of W and therefore, selectively reduce the work function of W (100) plane. When a sharp W needle terminated with (100) plane is used for the substrate onto which the ZrO is deposited, a field emitter with a low work function (2.6 eV) can be produced. (U.S. Pat. No. 3,374,386) It is generally believed that the reason for ZrO to form a stable thin film with (100) surface of W is related to lattice size matching between Zr atoms and W (100) plane lattice structure. Further findings suggested that W oxide can also form films over W (110) planes and (112) planes, which help W atoms to build up onto the tip apex oriented in the <100> or <111> direction when the tip is heated to a temperature to ensure high mobility of W atoms. The build-up tip is more inert than the original W tip without build up process. (U.S. Pat. Nos. 3,817,592, 7,888,654 B2) In these two cases, W oxide is not used to lower surface work function of the tip apex plane. Alkali-earth oxides, rare-earth oxides, thorium oxides, hafnium oxides are all known to be stable and has lower work function than ZrO and W oxide. In the best known prior art, no substrate material forms stable bonding with these oxides so that one can make a field emitter with reduced work function and increased surface inertness with those oxides.
As for making tip geometry with a raised apex, besides the above mentioned build-up process, another prior art (U.S. Pat. No. 7,431,856 B2) teaches that gas etchant preferably etches shank region over apex region of a tip under the application of an electric field. The described raised apex formation method only depends on the shape of the tip, not on the crystallinity nature or crystal orientation of the tip material.